An electrically conductive paste contains at least a metal powder and an organic vehicle. The electrically conductive paste has been widely used in the field of electronic components, since a conductor with a desired pattern or in a desired form can easily be formed by applying printing or the like to the electrically conductive paste. More specifically, the electrically conductive paste has been widely used for forming wiring conductors, such as conductor films and via hole conductors disposed in multilayer ceramic substrates.
As described above, the electrically conductive paste used for forming wiring conductors disposed on the multilayer ceramic substrate, in particular, the electrically conductive paste used for forming internal conductor films and via hole conductors located in the inside of the multilayer ceramic substrate, is co-fired in the firing step of sintering ceramic layers disposed in the multilayer ceramic substrate when the multilayer ceramic substrate is manufactured. In this case, with respect to the shrinkage behavior during the firing, it is desirable that the shrinkage behavior on the ceramic layer side and the shrinkage behavior on the wiring conductor side are matched with each other as much as possible.
However, in general, since the electrically conductive paste for forming the wiring conductor starts shrinking in a temperature range lower than the sintering temperature capable of sintering the ceramic layer, a difference in shrinkage behavior occurs between the ceramic layer and the wiring conductor. Consequently, a stress resulting from this difference in shrinkage behavior may cause an occurrence of structural defect, e.g., crack or delamination, in a produced multilayer ceramic substrate.
In order to overcome the above-described problems, for example, each of Japanese Unexamined Patent Application Publication No. 10-95686 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 9-295883 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 5-217421 (Patent Document 3) describes that the composition of an electrically conductive paste is improved and, thereby, an occurrence of sintering of the electrically conductive paste is suppressed.
Patent Document 1 describes an electrically conductive paste which is co-fired with a glass ceramic substrate and in which a specific metal oxide or a specific metal is added as a sintering inhibitor to Cu, Cu2O, a Cu—Cu2O mixture, or a Cu—CuO mixture serving as an electrically conductive component.
Patent Document 2 describes an electrically conductive paste which is co-fired with a glass ceramic substrate and which contains an electrically conductive component primarily containing copper. This electrically conductive paste contains an oxide of at least one of an alkali metal, an alkaline-earth metal, and boron as a metal oxide to decrease the viscosity of a glass component in the glass ceramic substrate and an inorganic material to retard sintering of copper.
Patent Document 3 describes an electrically conductive paste containing a copper powder and a glass powder.
However, the electrically conductive paste in each of the above-described Patent Documents 1 to 3 has the following problems to be overcome.
The electrically conductive paste described in Patent Document 1 contains the specific metal oxide or the specific metal in a state of a powder as the sintering inhibitor of the electrically conductive components. Such a sintering inhibitor cannot completely suppress the solid phase mutual sintering of electrically conductive component. Therefore, in order to satisfactorily suppress the sintering, addition of correspondingly large amounts of sintering inhibitor is required. Furthermore, since the sintering of the electrically conductive paste depends on glass diffusing from the glass ceramic substrate, unsintered regions may be left in the conductor in the case where the size of the conductor to be formed from the electrically conductive paste is large as compared with the diffusion distance of the glass or in the case where the amount of glass component diffusing from the glass ceramic substrate is not adequate.
The electrically conductive paste described in Patent Document 2 contains the oxide of at least one of an alkali metal, an alkaline-earth metal, and boron to facilitate diffusion of the glass component in the glass ceramic substrate. However, the electrically conductive paste described in Patent Document 2 cannot completely suppress the mutual sintering of the electrically conductive component as in the case of the electrically conductive paste described in Patent Document 1. Therefore, the shrinkage behavior thereof varies depending on the amount of addition of the inorganic material serving as a sintering-suppressing component or a dispersion state. Although the diffusion distance of the glass component in the glass ceramic substrate becomes longer by addition of an alkali metal or the like, the start point of the glass diffusion is essentially the interface between the conductor and the glass ceramic substrate, and the metal oxide to decrease the viscosity of the glass contributes to only the diffusion of glass from this interface. Consequently, in the case where the size of the conductor is large or in the case where the amount of glass component diffusible from the glass ceramic substrate is small, the effect is reduced and unsintered regions may be left.
The electrically conductive paste described in Patent Document 3 contains the glass powder. However, as in the case of the electrically conductive paste in the above-described Patent Documents 1 and 2, this cannot completely suppress the sintering of copper. Furthermore, in the case where the softening point of the glass component constituting the glass powder is at least 300° C. lower than the firing temperature applied to the firing step for producing the ceramic substrate, the sintering behavior of the electrically conductive paste is not varied and, in addition, sintering of the ceramic substrate occurs at a temperature lower than a predetermined sintering temperature by diffusion of softened glass into an unsintered ceramic substrate and a plurality of regions having mutually different sintering temperatures are formed in the ceramic substrate, so that this may cause an occurrence of cracking and the like. Since the surface of the copper powder exhibits inherently poor wettability with general glass, the softened glass is pushed out to the grain boundary portion during the mutual sintering of the copper powder. As a result, large amounts of glass component may seep to the surface of the conductor. The thus seeping glass component exerts an adverse effect when a surface treatment, such as plating, is conducted, for example.
On the other hand, the method in which a so-called non-shrinkage process is applied has been noted as a method for manufacturing a multilayer ceramic substrate. This can respond to requirements for miniaturization, higher function, higher precision, higher density, and the like of the multilayer ceramic substrate. Consequently, distortion and variations in shrinkage due to sintering in the firing step conducted in the manufacture of the multilayer ceramic substrate are suppressed, and different types of materials can be incorporated into the multilayer ceramic substrate.
Some types are included in the non-shrinkage processes. For example, there is a process, in which a constraint layer containing inorganic material powder having a sintering start temperature or a sintering finish temperature different from that of substrate ceramic green sheets to become a plurality of ceramic layers included in the multilayer ceramic substrate is disposed along each of the substrate ceramic green sheets, the shrinkage suppression effect of the constraint layer is allowed to be exerted on the substrate ceramic green sheet, that is, the ceramic layer, in the firing step, and thereby, the non-shrinkage process is achieved. This may be referred to as an interlayer constraint method, and is particularly effective at suppressing the distortion in the thickness direction of the multilayer ceramic substrate and suppressing the distortion at a cavity portion in the case where the cavity is formed in the multilayer ceramic substrate.
In addition, the non-shrinkage processes also include a method, in which constraint layers that do not shrink during the firing are disposed along both main surfaces of a green multilayer ceramic substrate and, thereby, shrinkage of the multilayer ceramic substrate in the firing step is suppressed. In this method, a porous ceramic layer that is not sintered at the firing temperature of the multilayer ceramic substrate or a conductor film, e.g., metal foil, is used as the constraint layer. If necessary, the constraint layers are removed after the firing step.
When the multilayer ceramic substrate provided with via hole conductors disposed penetrating specific ceramic layers is manufactured, the electrically conductive paste for forming the via hole conductors is co-fired in the firing step to sinter the ceramic layers, as described above.
When the above-described non-shrinkage process is applied to the above-described manufacture of the multilayer ceramic substrate, in the firing step, the ceramic layer exhibits the shrinkage behavior substantially only in the thickness direction since the shrinkage in the main surface direction is suppressed. The degree of the shrinkage in the thickness direction is larger than the degree of shrinkage in the case where the non-shrinkage process is not applied. On the other hand, the electrically conductive paste for the via hole conductors exhibits the shrinkage behavior isotropically. As a result, inconveniences shown in FIG. 4 may occur.
FIG. 4 is a magnified sectional view of a part of the multilayer ceramic substrate 1 after the firing. This multilayer ceramic substrate 1 is an example of those manufactured by applying the non-shrinkage process through the use of the above-described interlayer constraint method, and a constraint layer 3 is disposed along each of a plurality of ceramic layers 2. A via hole conductor 4 is disposed penetrating specific ceramic layers 2.
As described above, in the firing step, the ceramic layers 2 exhibit the shrinkage behavior substantially only in the thickness direction, whereas the via hole conductor 4 exhibits the shrinkage behavior isotropically. Consequently, the ceramic layers 2 are peeled from the via hole conductor 4 at the outer perimeter portion of the via hole conductor 4, and gaps 5 may occur there. The end portion of the via hole conductor 4 may protuberate from the main surface 6 of the multilayer ceramic substrate 1, and a relatively large protuberance 7 may be formed around the via hole conductor 4.
Usually, the electrically conductive paste to become the via hole conductor 4 starts shrinking in a temperature range lower than the shrinkage start temperature of the ceramic layer 2 due to sintering. Consequently, the shrinkage behaviors of the ceramic layer 2 and the via hole conductor 4 do not match with each other. This also causes the occurrence of the above-described gap 5 and, in addition, this causes the occurrence of structural defect, e.g., crack or delamination, in the multilayer ceramic substrate 1.
It is believed that inconveniences encountered in the method for manufacturing a multilayer ceramic substrate through the use of the above-described non-shrinkage process can be overcome to some extent by realizing a technology capable of optimally controlling the shrinkage behavior during firing of the electrically conductive paste used for forming the via hole conductor 4. In this regard, the above-described inconveniences are not satisfactorily overcome by the technology disclosed in each of the above-described Patent Documents 1 to 3.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-95686    Patent Document 2: Japanese Unexamined Patent Application Publication No. 9-295883    Patent Document 3: Japanese Unexamined Patent Application Publication No. 5-217421